C-protein is a minor component of the contractile elements of muscle but its physiological role is unknown. It interacts strongly with myosin, which correlates with its structural localization on the thick myosin-containing filaments in skeletal muscle. However it is also capable of interacting with actin, and the binding of C-protein to the actin in intact thin filaments in muscle is regulated by calcium ions as is the contractile cycle of muscle. When C-protein is added to actin or myosin filaments in vitro, it significantly modifies their interaction as manifested by ATPase activity measurements. In the coming period of this project we plan to explore further the relation of C-protein effects to the calcium control of actomyosin systems in vitro and also various aspects of the structural interaction of C-protein with myosin filaments. In addition, we are exploring the distribution of C-protein among different kinds of mammalian muscles and non-muslce tissues, using immunochemical techniques.